The present invention relates to graphics plotters and, more particularly, in a graphics plotter having a penholding mechanism upwardly spring biased and slidably and rotationally mounted on a beam for longitudinal positional movement along the beam and rotational movement around the beam to affect raising and lowering of a pen held by the penholding mechanism under the control of plotter control logic, to the improvement to provide more accurate raising and lowering of the pen comprising, an actuator rod disposed parallel to the beam for rotational movement about a longitudinal axis; linkgage means for slidably linking the actuator rod to the penholding mechanism so that as the penholding mechanism is moved longitudinally along the beam rotation of the actuator rod will cause corresponding rotation of the penholding mechanism; and, powered rotation means connected to the plotter control logic to receive control signals therefrom for accurately rotating the actuator rod in small increments in response to the control signals whereby the penholding mechanism can be rotated to raise and lower a pen held therein accurately in small increments; and to the improvement for providing a feedback signal related to the rotational position of the pen comprising, transducer means operably connected to sense rotation of the actuator rod and develop a signal at an output thereof related to the rotational position of the actuator rod.
The essential elements of a graphics plotter are depicted in simplified form in FIG. 1. A penholding apparatus 10 holding a pen 12 is moved along a beam 14 from side to side as indicated by the arrows 16 over paper 18 to create one axis of the drawing while the paper 18 is moved longitudinally under the beam 14 to create the other axis. The pen 12 is raised and lowered as indicated by the arrows 20 to affect writing and non-writing movement of the pen 12. Prior art approaches to pen raising and lowering (also referred to as "dropping") fall into two general types as shown in FIGS. 2-3 and FIGS. 4-5, respectively. In the prior art technique of FIGS. 2-3, the penholding apparatus 10 is free to slide longitudinally on the beam 14 only. The pen 12 is held by a vertically slidable member 22 and biased to the raised position of FIG. 2 by a spring, or the like (not shown). A solenoid actuator 24 is connected by an armature linkage 26 to the slidable member 22. By applying an electrical current to the solenoid actuator 24, the pen 12 is lowered by sliding the member 22 vertically downward as shown in FIG. 3. When the electrical current is removed, the member 22 springs vertically upward under the bias force, taking the pen 12 with it. Thus, the mechanism of FIGS. 2-3 affects what is commonly referred to as "bang bang" control; that is, the pen 12 is banged between the two extremes of raised and lowered by the bias force and solenoid actuator 24, respectively.
The other common prior art approach to pen raising and dropping is shown in FIGS. 4 and 5 wherein the penholding apparatus 10 is both slidably and rotatably mounted on a cylindrical beam 14'. An actuator rod 28 is rotatably mounted on a bar 29 parallel to the beam 14'. The actuator rod 28 has an actuating ridge 30 extending therefrom and running longitudinally along which the penholding apparatus 10 slides as it moves along the beam 14'. As with the prior art of FIGS. 2 and 3, the penholding apparatus 10 is rotatably biased to the raised position of FIG. 4 and bearing against the actuator ridge 30 by means of a spring 31. A solenoid actuator 24 is connected by an armature linkage 26 to rotate the actuator rod 28 when energized. Thus, as shown in FIG. 5, to drop the pen 12, current is applied to the solenoid actuator 24 causing the armature linkage 26 to rotate the actuator rod 28 as indicated by the arrow 32. This, in turn, causes the actuating ridge 30 to rotate the penholding apparatus 10 about the beam 14' against the bias force of the spring 31 as indicated by the arrow 34, thus rotating the pen 12 to its dropped position. It should be noted that the rotational aspect of the pen dropping movement are exaggerated for purposes of the drawing.
There are instances wherein it would be desirable to have greater control over the pen 12 with respect to its raised and lowered position. Solenoid operation as in the above-described prior art approaches to pen control provides neither movement control nor positional feedback information.
Wherefore, it is the object of the present invention to provide apparatus for continuously controlling the up and down movement of a pen in a graphics plotter.
It is a further object of the present invention to provide positional feedback information relative to the up and down position of the pen in a graphics plotter enabling more optimum pen control to be exercised by the plotter control logic.
Additional objects and benefits of the present invention will become obvious from the description contained hereinafter taken in combination with the drawing figures which accompany it.